The devices for heating and/or frothing milk are well known.
For instance, a device for preparing frothed beverages to be used in conjunction with a machine for preparing coffee and other hot beverages is known from document EP 0607759 A1.
The prior art device comprises a nozzle, which is controlled by a manual valve and which has a first chamber for steam inlet and a second chamber for steam outlet. The two chambers are connected through a passage, of reduced cross-sectional size, where a hole communicating with the surrounding environment is provided.
Steam flow through the passage with reduced cross-sectional size causes, by Venturi effect, a sufficient pressure decrease to ensure intake from the surrounding environment of air which, mixed with steam, exits through the second chamber and is introduced into the liquid, e.g. milk, frothing it.
A first problem of such a prior art is that the nozzle is effective for preparing frothed beverages, but it is ineffective for preparing non-frothed hot beverages, since the passage with reduced cross-sectional size generally entails either introduction of both air and steam into the beverage, thereby frothing it, or the need for an accurate control of the manual valve to limit steam inlet and the consequent air admission by Venturi effect.
Of course, vapour inlet control entails the further problem that the time necessary for preparing heated beverages is considerably higher than that required for preparing frothed beverages.
An automated machine for preparing espresso coffee, comprising a device for producing hot or frothed milk to be added to the espresso coffee, is known from document EP 0472272 A2.
The device of the prior art machine comprises two valves for steam introduction through a duct located upstream of a chamber with narrower cross-sectional size than the duct: The chamber, operating by Venturi effect, comprises an orifice for air intake from the surrounding environment. The two valves in the device have different flow rates.
Consequently, opening the valve with higher flow rate results in a greater steam flow into the chamber and hence enables preparing frothed milk, since pressure reduction in the chamber by Venturi effect is high.
Closing the valve with higher flow rate and opening the valve with lower flow rate on the contrary enables preparing hot milk, since pressure reduction in the chamber by Venturi effect is minimum and hence insufficient to enable air flow from the surrounding environment into the chamber.
Such a prior art, while enabling preparing both hot and frothed milk, has in any case the problem that preparing hot milk generally requires a very long time, in any case longer than that required for preparing frothed milk.
As the skilled in the art readily realises, such problem is very annoying in public places, where the beverage preparation time must be extremely short to provide for both short service time and good clients' satisfaction.
Essentially, the Applicant has realised that devices ensuring extremely reduced preparation time, especially in respect of hot beverages, irrespective of whether hot or frothed beverages are to be prepared, are not commercially available.